So what you can do is perform contrast-detect AF in liveview, then switch to non-liveview and compare focus accuracy.

Other things to remember: disable (or set to minimum) the in-camera sharpening. It tends to partially negate the lens differences. Don't worry that images will look a bit soft from both lenses when sharpening is set to minimum. It's the comparison that counts.

Then, try the upside-down test also: shoot images with the camera upside down and also normal and compare whether these images are sharp in the same place and soft in the same place (after rotation). The lens with which these differences are minimal or non-existent has the best centered lens elements.

Thanks for the correction on the contrast AF on the XSi. I don't own one and breifly scanned the specs here on DPR and did not see that listed. I agree with your test for AF calibration as that is my standard too.

I've never done the upsidedown test for decentering and can understand how it would work for a lens with a tripod collar but how do you perform the test for a lens without? The only way I can think of is if one has either a special bracket or a tripod that allows the center post to be removed and inserted from the bottom and even then it would appear alignment could be an issue. I'll have to try this and see how it works.

Another method is to position the camera in front of a post, tower, face of a building with detail, etc. and focus precisely with the camera level in both planes. Then swing the camera so the area of original focus is at the left side of the frame and take an exposure and then do the same for the right (exposure should be in manual). The left and right images should be virtually identical in sharpness and if not, the lens probably has some decentering.